Since the first meeting of the International Workshop on the Fragile X syndrome and X-linked Mental Retardation in Bethesda in 1983 there has been a steady growth of information related to these disorders. For this reason a forum is needed to bring together scientists and clinical investigators for discussion and exchange of ideas. The proposed workshop in 1989 will focus on molecular and family studies since the rapidly evolving technology has yielded new information. Advances on other important topics, notably the nosology of X-linked mental retardation, clinical aspects of the phenotypic and behavioral aspects of fragile X syndrome (fra(X)), cytogenetics, prenatal diagnosis, epidemiology and genetics, and linkage, will be reviewed and hypotheses and future research issues discussed. The proposal requests funds to pay the registration fees of promising junior American investigators whose presentations will be chosen on a competitive basis. Funds are also requested to pay for the travel of distinguished scientists to address plenary sessions, as well as the leaders of the workshop who will incur significant expenses for their work prior to or immediately following the workshop. The travel awards will be for plenary session speakers who normally would not attend this meeting, and for persons who have been asked to lead sections or serve on key committees, but whose institution or grants cannot provide full support. If this application is funded, it will make a major contribution to the success of the 4th International Workshop on the Fragile X syndrome and X-linked Mental Retardation.